Yes,
you can do this by selecting the Flowers category (left click on it) and exlude the Blue category (to do this, you just have to right-click on the Blue category and select 'Hide <Blue> from the current set')

Attention ! if 'Blue' is not the last category of the tree (if it has subcategories such as 'Dark Blue', 'Pale Blue' ...) the result is not working at all ! (I hope it would be corrected in the v3.0...)